cpufreq: dt: Add support for hi3660

Add the compatible string for supporting the generic device tree cpufreq-dt
driver on Hisilicon's 3660 SoC.

Signed-off-by: Tao Wang <kevin.wangtao@hisilicon.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>